The Institute of Electrical and Electronics Engineers (IEEE) has adopted a set of standards for wireless local area networks (LANs), known as 802.11. Wireless products satisfying 802.11a, 802.11b, and 802.11g are currently on the market, for example.
Recently, an 802.11n standard, known also as the Enhancement for High Throughput wireless standard, has emerged. Under the 802.11n standard, transmitters and receivers each have multiple antennas for transmission and reception of data. As a multiple input, multiple output (MIMO) technology, 802.11n is designed to coordinate multiple simultaneous radio signals, and is expected to support a bandwidth of greater than 100 megabits per second (Mbps).
Under 802.11, a device (or client), also known as a station (STA), accesses a wireless LAN (WLAN) by following a communication protocol, essentially involving the transmission of frames to an access point (AP). The frames, or packets, may be data frames, control frames, or management frames. The frames convey information that enables the AP to “connect” the device to the WLAN. The APs and STAs in a WLAN constitute a wireless neighborhood.
The 802.11n specification defines two parallel types of packet aggregation: aggregate MAC service data unit (A-MSDU) and aggregate MAC protocol data unit (A-MPDU), where MAC is shorthand for medium access controller. The A-MSDU and A-MPDU may be used separately or together. A MAC service data unit (MSDU) is the frame entering or exiting the top of the MAC. A MAC protocol data unit (MPDU) is the frame entering or exiting the bottom of the MAC.
The aggregate MSDU, A-MSDU, aggregates data MSDUs into one MAC protocol data unit, or MPDU. Each such packet included in the MPDU contains a short header with the byte count, the destination address, and the source address. The aggregate MPDU, A-MPDU, aggregates MPDUs into one PHY protocol data unit, or PPDU, with PHY being shorthand for the physical layer of the stack.
A merger of both aggregation types, A-MSDU and A-MPDU, means that an MPDU containing A-MSDU is included in A-MPDU. Both types of aggregation are mandatory in the current draft of the 802.11n specification. The receiver, or recipient, is therefore expected to support both types of aggregation when prompted by the transmitter, or originator.
Under 802.11n, the maximum size, known as the maximum transmission unit, or MTU, of A-MSDU is approximately 4 Kilobytes (Kbytes). The MTU that can be sent by MPDU is 1500 bytes. To support A-MSDU, the recipient has to be able to receive the full-sized A-MSDU. In other words, the recipient must allocate memory to receive the A-MSDU packet.
There are two ways to transmit an A-MSDU packet. The packet may be sent under the normal acknowledge (ACK) protocol. Alternatively, the packet may be transmitted using a block ACK agreement. An A-MPDU, by contrast, mandates using a block ACK agreement.
To use or not use either type of aggregation, A-MSDU or A-MPDU, is entirely up to the originator, the station that transmits the data. The station that receives the data, or recipient, should be prepared to receive the data, whatever the aggregation types is. A station that receives an “add block ACK (ADDBA) request” should allocate a block ACK (BA) reordering buffer. ADDBA request is the name of the action frame that is used to establish the block ACK agreement. A BA reordering buffer contains several buffers, in which each one is of the MTU for A-MSDU size (e.g., ˜4 Kbytes). This must be done by the recipient station, even when the originator has no intent to merge A-MSDU into A-MPDU. Thus, even where the originator does not send the A-MSDU packets aggregated into an A-MPDU packet, the recipient has allocated the larger MTU size for each buffer in the BA reordering buffer. In such a situation, more than half of the memory allocation in the recipient buffer is wasted. That is, 4000 Kbytes−1500 Kbytes, or 2500 Kbytes, of each buffer is wasted.
Thus, there is a continuing need for a packet aggregation method that is efficient for the recipient while supporting the 802.11n specification.